Two clocks in different strength gravity will go out of sync.
If a strong pulse passes by, and it was shaped so that one clock spent time in stronger gravity than the other, then they'd go out of sync. So it is possible ... the short answer to your question is "probably".
You'd have the usual trouble working out whose clock was "correct" (answer: neither of them) but you'd theoretically see the difference.

Imagine 2 clocks emitting light pulses at a fixed distance. I think, they will see each other periodically ticking slower, rep. faster, due to the locally expanding and contractiing space-time in case a gravitational wave passes by.

Staff: Mentor

Length changes are easier to detect - we have gravitational wave detectors sensitive to relative length changes of 10-22 - less than the diameter of a proton over a kilometer. That is more precise than our best clocks (~10-17).

"The Milky Way has a black hole at its core with a mass of four million times that of the Sun.

A billion solar mass black hole would have an event horizon 3 billion km in radius — roughly the distance of Neptune to the Sun.

See where I’m going here? If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!

That, to me, is by far the oddest thing about black holes. Sure, they warp space, distort time, play with our sense of what’s real and isn’t… but when they touch on the everyday and screw with that, well, that’s what gets me."

"The Milky Way has a black hole at its core with a mass of four million times that of the Sun.

A billion solar mass black hole would have an event horizon 3 billion km in radius — roughly the distance of Neptune to the Sun.

See where I’m going here? If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!

That, to me, is by far the oddest thing about black holes. Sure, they warp space, distort time, play with our sense of what’s real and isn’t… but when they touch on the everyday and screw with that, well, that’s what gets me."

I had a big argument about that in these forums a while ago ... the trick to thinking about this sort of thing is to ask "who is doing the measuring?" Would an infalling observer ever see the gas cloud?

Remember also that the physical singularity at the center of a black-hole is most likely an artifact of our maths rather than something that actually happens.

There again since they are both black holes perhaps nothing gets ejected or emitted and they just join and become one?

I think it depends upon whether or not any matter gets caught up in the merger. But the gravity wave emissions of the event would be quite massive. I'm not sure how noticeable it would be, but obviously our gravity wave detectors would go nuts.

Staff: Mentor

The gas cloud would collapse and form a star long before you have 3 billion solar masses inside. If that is not possible (because you use iron or similar elements where fusion does not release energy), you quickly get a white dwarf, then a neutron star, and then a smaller black hole, slowly growing as you put in more gas.

Staff: Mentor

In principle, you can probably shoot gas in from everywhere at the same time in the right way to avoid collisions, assuming magic outside controlling all that mass. Just slowly pumping mass inside (and 1 solar mass per second is slow here!) won't work, however.

1 solar mass per second needs a flow velocity of 100km/s.
100 solar masses per second need 10000km/s... ;).

... A billion solar mass black hole would have an event horizon 3 billion km in radius — roughly the distance of Neptune to the Sun. ... If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!

Why would that be a black hole? I assume that we would have to fill the sphere with a billion solar masses of compressed air to have that effect ... wouldn't we?